A common practice in liquid chromatography is the so-called gradient elution where the composition of the solvent is varied under control during the separating process. The variation of the solvent composition as a function of the time is called the elution gradient. Since this gradient often has a substantial influence on the separation characteristic and the analysis result, exact repeatability of the gradient is required.
Typically, two pumps are used for producing an elution gradient, each pump delivering a solvent component to a mixing chamber. Both pumps are controllable with regard to the delivered volume per unit of time. Variation of solvent composition is achieved by varying the flow rates of the two pumps relatively to each other. An example for a gradient elution system of this type is given in U.S. Pat. No. 446,293. However, a gradient elution system of this type requires pumps with flow rate adjusting ranges which are extremely difficult to realize. In a mixture of two solvents e.g. a variation between 1 and 99% concentration of each component is required. Additionally, the flow rate through the column should be variable, e.g. from 0.1 to 5 ml/min. With flow rates below approximately 1 ml/min, however, it is hardly possible to exactly control the solvent composition over a great adjusting range. On the other hand such small flow rates are desirable in connection with modern efficient short columns.
In another prior art apparatus for producing an elution gradient, described in U.S. Pat. No. 3,985,019, the solvent components are supplied to a single plunger pump via a quickly operating open/close valve for each component. These valves are opened and closed alternately during the intake stroke of the pump. The solvent composition is determined by the relative opening ratio of the valves. However, since solenoid operated valves have a limited switching speed, this technique can be used only in connection with relatively slow plunger pumps with a stroke frequency below 1 Hz. The stroke volumes of such pumps are typically from 50 to 200 .mu.l. Thus, adverse conditions arise with slow flow rates, since the solvent composition can be varied only in steps with a duration of at least the duration of 1 stroke. With a flow rate of e.g. 200 .mu.l/min and a stroke volume of e.g. 100 .mu.l not more than one step in solvent composition is possible per half minute.